Alloy resistance element and method for manufacturing same



Feb. 19, 1952 E, WEBER ETAL ALLOY RESISTANCE ELEMENT AND METHOD FOR MANUFACTURING SAME Filed Sept. 26, 1946 .JR MM Mm m A n Patented Feb. 19, 1952 2,586,752 ALLOY RESISTANCE EIiEMEN'r' AND METHOD FOR- MANUFACTURING lSAME Ernst Weber, Mount Vernon, and Stanley Adams Johnson, New York, N. Y., assignors'to Poly-v technc Institute of Brooklyn, Brooklyn, N. Y.,.

a corporation of New York Application September 26, 1946, Serial No. 699,546

, 1 This invention relates to the formation of metallic lms by thermal evaporation, and it is 4concerned with both the method and the apparatus used in the production of such films, and with the Vresulting product.

The present invention was developed especially for the production of thin metallic films on dielectric carriersffor use as electrical resistance elements, especially in applications involving high frequency and ultra-high frequency currents. It will bezunderstood, however, that the invention may beLused for the formation of metallic films for any purpose.

"`-Specically the problem back lof the present invention was that'of producing resistance elements of rugged construction and having highlystable' characteristics adapting them for use in the precision measurement and control of currents of extremely., high frequency. Resist-` 'ance elementsxfor this purpose must be formed of metallic films having the characteristics of excellent resistance stability, frequency insensitivity, low temperature coefficient, and uniformity. The film must also be resistant to mechanical abrasion and oxidation, and must be temperature stable and unaiected by humidity.

Such resistance elements are especially useful as'attenuator units. which dissipate some of the electric energy inthe form of-joule lossesin the metal film. For this purpose, itis important that the lm be ofvery small thickness, preferably less than the depth of penetration of the current, and this requirement involves certain diiculties as will be explained later.

It has long been known that `thin metallic films .could be [formed on dielectric carriers by thermal evaporation of the metal to be deposited and allowing the vapor of the metal to condens on the relatively cool surface of the carrier: In attempting to develop electrical resistance elements by this method, in which the metal to be evaporated was placed on a heater filament, applicants soon discovered that thin' metallic lms formed in this manner may have a 'number of undesirable characteristics whichv render them unsuited 4for the purposes referred 't'oabove For example, it was found that depending upon the conditions under which the lm was formed, there 'might be aging eiiects in which there is a relatively slow change in resistance value of the 4film with', time; lunder certain conditions, espe'- ci'ally wherevit'was attempted to 4buildfup` greater y'than fa certain thickness;I therewasta tendency'for the'lms to ake or peellfrom-the carrier surface;' fllms' formed from puremetals 9i Claims. (Cl. 117-47) have resistance values which varied with teniv perature; the material of the vaporizing heater," lament may adversely affect the iilm; andvif the lm is extremely thin it will have unstable resistance. C

The present invention involves the process and apparatus by Which-itis possible to produce very thin metallic films-of very stable characteristics` and of rugged mechanical construction. thermore, the process produces 'uniform' vresults in successive operating cycles.

In producing-,resistance elements for theA pu` poses indicated above theiilms should be free from oxidation or corrosion when'exposed to air; This requirement naturally vsuggests the usefof noble metals such as gold and'platinum for the ilm material, but it wasfound'that in lms produced fromA these'metals having a reasonable amount of resistivity, the thickness of the film is onlyl a few times greater than the molecular diameter, and the resistance of the l'm is unstable. In order to obtain a film of therequired thickness for resistance stability, and to be free from variations with temperature, the lm material is formed of Itn alloy having considerably higher resistivity than the noble metals and having a low temperature coefcient. Applicants discovered that for use in the evaporation process; the alloys must bevformed of component met-:- als having approximately the same melting point if the resulting lm is to have the same electrical characteristic as that of the alloy. Alloys suitablefor this purpose are those commonly known as lNiclfiromje (nickelf and chromium), f Constantan (nickel and copper) and Lucero" .(copperand nickel), Inprder to obtain stability ofresistance value, the film thickness must be of the order of `50 -m'olecules or greater. For "Nichrome, which is the preferred alloy, the lin -should have a thickness of the order of angstrom units or greater. In order Lto insure against the eiects ofoxida- `tion andxcorrosion, a; protective coating is applied'. to the .resistance unit immediately after the* metal Yfilm is `formed and While the unit is 'still under vacuum.v 'This coating mafy be formed 'by' evaporation of quartz, but the preferred material for the coating ismagnesium fluoride'. A`A 'coating of this material, about tenor `morenrnil- 4lionthsV o f an finch thick, notfonly protects' the iilmrom chemical deterioration,- but also 'preventsmechanical injury tothe resistance film: by reason ofthe great hardness and strength of tl'iefluoridecoating-- This method of applying 3 the protective coating Aalso preserves the low temperature coefficient of this alloy lm.

Applicants have discovered that in producing resistance elements having the characteristicsreferred to above. the following five conditions must be satised in the formation. of a film of. NichromeV evaporated from a tungsten filament:

l. The dielectric carriers to be coated, usually in the form yof glass plates or rods, must be thorf oughly cleaned of all contaminating substance of any sort such as dust, lint, and any organic or inorganic deposits of any typev which might pre'- vent strong bonding or adhesion of the lm hto the carrier.'V Even a slight surface contamination will cause considerable non-uniformity of the iilm. The vaporizing apparatus should also be cleaned of contaminating substances.

2. During the formation of the film on the carrer..the carrier should be maintained at a temperatureV not less than 300"A C. and not appreciably greater than 350 C; This condition is necessary jto secure a strong bond between the nlm and the carrier. Satisfactory adhesionv of the nlm to the carrier is not obtained for temperaturesbelow this range andv the lm is not uniform and is subjectto aging. The aging eii'ect is probably due to the reorientation of fthe crystalline batches of metal particles whenv the batches are heated by an electric current passing through them. The proper heating of the carrier during evaporation permits the required orientation to take place while the metal is being deposited.

Insuiilcient heating' of the carrier during evaporation also results in a considerably higher teiriperatureV coeflicient ol?v resistance of theltllm.

If the carrier is heated higher than 350' C; Igases released from the metal parts of the apparatus v prevent the formation of smooth, uniform films. Also, the heater wires used to heat the carrier parts may veven start to emit and deposit a metal film on the reverse side of the carrier parts'.

3. Where the metal being evaporated contains an element which has a dissolving effect on the vaporizing iilement, there should'be an adequate supply of' coating metal on the filament tocom- "pletely cover the filament during the entire cycle of evaporation. If' an adequate supply ofjcoating metaljis not provided, the coating metalvvwill becomevcontaminated by the dissolved metal-of the filament,l anda strong aging effect results. In the case of Nichrome applied: to a tungsten lament',vthe nickel tends to dissolve the tungsten. If too much "Nichrome is present on the iilamentrduring evaporation, there is danger that the iilament may bev Weak-cned byv the dissolving of 'the tungsten to such extent that the filament will break. Thespecific amount of Nichrome." employed in the process will be explainedflater.

4. The speed of evaporation and the time of the vaporation cycle are also important factors.

The speed of evaporation is necessarily dependent upon the temperature of the heater filament. If the temperature of the filament is too low,' the metal molecules, having lower speed,

v.Inayfnot form a strong bond with the carrier and theiesulting filament becomes mechanically unstable. At higher. temperatures, they released molecules ofl Nichrome travel at higher speed yand penetrate into the outermost molecular layer -of the carrier and -form a strong bond resulting in 'a smooth anddurable. film. The actualA temperature'of the filament during evaporation has 4 not been determined, but the filament employed in applicants work consists of three 20 mil tungsten vwires plaited together and carrying a total current of 30 amperes which should produce a temperature in the neighborhood of 1300 to 1400 C.

With regard to the time for each lament evaporation cycle, considerable experimentation has shown that evaporation from a single vaporizing filament should not continue for a period longer than three minutes.v A longer time of evaporation is likely to cause contamination of the deposited ill'm by dissolved tungsten from the heater filament.

Where a film thickness is required greater than that which can be obtained by the evaporation from a single filament in a three-minute period,.it is necessary to build up the film by the evaporation from a number of filaments energized in succession. Also, we have found that it is not possible to use a tungsten filament for' more than one evaporation cycle in evaporatiug Nichrome 5. The bell jar in which they evaporation takes place must be maintained at the proper pressure during the formation of the film. This pressure must be less than 10-4 mm. of mercury in order to obtain mechanically strong, non-aging iilms. For consistent results and uniform films a pressure of 6 or 7 105 mm. of mercury or less should be used. At lower pressures, the mean free path ofthe molecules is considerably greater and. therefore, the kinetic energy is greater, thus resulting in a stronger bond. In the apparatus employed, the mean free path of the metalmolecules is about twice the maximum distance ot travel of the molecule. The chance of collision with remaining gas molecules is considerably reduced, and most of. the metal molecules will keep-v their high kinetic the carrier. I

Suitable apparatus for carrying out the present invention is illustrated in the accompanying drawing in which:

Figure 1 is a side elevational view showing the apparatus enclosed Within a vacuum bell jar, certain parts being shown in vertical section;

Figure 2 is a plan view of Figure 1v with the heatinghood andfthe carrier supporting jig removed from` the vertical standards; f

Figure 3 is a view'of the heating hood from the underside thereof;

Figure 4 isa top plan View of the carrier supporting jig; l

Figure 5 is an enlarged plan View of a test strip employed for determining the thickness ofthe deposited nlm during evaporation; and

Figure 6 is an end view of the test strip showing energy until they ,strike a connector applied thereto.

Figures 3 and 4 are on a somewhat larger scale than Figures 1 and 2 and Figures 5 and 6 are'on a still larger'scale. The various figures of the drawing are somewhat diagrammatic. r:

The apparatus consists of a bell jar construction such as a glass bell l shown in vertical section mounted above the center of base plate 2 and-'carries on the upper face. thereof a plurality of vertical terminal posts. A pair of these terminal gasolina posts arranged on opposite sides of the plate 3 support a horizontal heater filament 4. Similar heater filaments 5 and 6 Vare supported in closely spaced parallel relation with lament 4 by corresponding pairs of terminal posts. Two addi- 5 tional pairs of terminal posts support two horizontal filaments which carry cups 'I and 8 for vaporizing the material to form the protective; coating. One terminal of each pair of filament'4 terminals is grounded to base plate 2, and the rem maining terminals for the different laments are connected respectively to insulated terminals 4a, 5a, 6a, 1a, 8a mounted on base 2 within the jar I.

Surrounding the terminal plate 3 and extendvV ing upwardly is an open ended glass cylinder Ill ig (shown in section in Figure 1) provided to protectv` the jarI and other parts of the apparatus from ,y molecular radiation from the metal being evapov-, rated and from the filaments. This protective; cylinder prevents the formation of a metallic iilm 20 on the interior of the jar I and greatly reduces'iv the labor in. keeping the interior Wall of the jar Y clean.

A carrier supporting jig is supported at theg upper end of rods 2a, 2b, 2c, 2d. The jig has a'fgg, main frame II of rectangular shape, as shown in@ Figure 4, and is provided with perforated ears at the corners through which the reduced ends y of the supporting rods extend. Two curved rails I2 and I3 are supported on frame II with the" ends resting upon frame sides Ila and I Ib. re-k-j-"- spectively. The frame sides IIa and IIb havev secured to their outer edges vertical strip 'IIa' and I Ib which extend above the sides I Io; and I Ibi and are provided with longitudinal slots througligg, which suitable screws pass and have threadedA engagement with the ends of rails I2 and I3. By'f this construction it is possible to adjust the* distance of separation between the rails I2 and I3 to adapt these rails to support carriers of dit-,fw ferent lengths and to clamp the rails in adjusted position. In Figure 4 of the drawing, the two rails I2 and I3 are shown supporting eight carf" rier pieces which usually are in the form of flatA glass plates shown in dotted lines at I4, four pieces arranged on opposite sides of the centern 45 lof the rails. Suitable metal strips |2a, I2b and I3a, I 3b are' ,adjustably secured to rails I2 and I 3, respectivelyf'- and serve as stops to prevent longitudinal movef-V v ment of the glass plates I4 when they are mounted on the rails. By adjusting the position of these; end stop strips on the rails I2 and I3, it is pos-A sible to adjust the extent of the end portions oi" the plates I4 which are shielded by the rails I2 and I3. 5 4As will be seen from Figure 1, the rails I2 and I3 'are of arcuate shape and the center of curva-f ture of these rails is positioned substantially at' the filament 4, thus supporting the plates I4 at equal 'distances from the evaporating lament.

The distance from the lament to the plates being have threaded engagement with the rails I2 and 70 I3 as shown. The purpose ofthe strip I5 is to vhold the center portions of the two rails in proper Yspaced relation.

A Supported immediately beneath the spacer strip `I5 at the center of the carrier jig, and at the :s

same distance from the lament as the plates I4, is a test strip I6 which is illustrated in greater detail in Figure 5. This test strip is made of the same material as the plates being coated and preferably is of the same width. Terminalpor-4 tions I6a and Ib of the test strip are provided with metallic coatings of low conductivity, and these terminal coatings are spaced apart a known distance I6c convenient for determiningv the thickness of the lm during evaporation. These terminal coatings may be formed by applying a coating of gold-platinum paste to the end portions of the strip and reducing these coatings to an adherent metallic coating by the usual burning-on process employed in the art of metallizing glass. The terminal portions I 6a and |611l are connected to suitable electric terminals I6a' and Ibl' mounted on frame II, and the connections to the terminal portions of' the strip are secured by means of a special clamp illustrated in Figure 6. This clamp involves a single metal strip'. I1

having the outer end thereof bent into a V forma-- tion with the outer leg I 'Ia of the Vv bearing against the upper face of the plate I6, and the other leg I'Ib extending underneath the plate and having a clamping screw I8 threaded therein and extending into engagement with the terminal coating on the plate.

One of the terminals of the test strip mounted on frame I I is grounded to the base plate 2 while the other terminal is connected to. an insulated terminal |60 mounted on plate 2 as shown in Figure 2. This terminal is connected to a suitable resistance measuring instrument such as a Wheatstone bridge diagrammatioally represented at I9 in Figure 2. By this arrangement it is possible to measure the resistance of the test strip I6 at any time during the evaporation process.

Supported immediately above the plate jig on vertical rods 2a-2d is a heater unit for maintaining the carrier plates I4 at a predetermined temperature during the coating process. vThis heater unit is formed of a metallic box 20 of rectangular shape and mounted in an inverted position above the carrier jig by two end pieces 20a and 29h provided with four holes which receive the reduced extensions on the upper end of rods 2a, 2b, 2c and 2d. The box or hood 20 preferably is formed of sheet copper, and the bottom of the box is of arcuate shape curved about filament 4 as a center, as shown in Figure 1. A plurality of insulating rods 2I are mounted A,within the box 20 transversely thereof and in an arcuate formation, see Figures 1 and 3. These rods serve 5 to support a heating filament 22 which is arranged within the hood in the manner shown in Figure 3 and is connected to two insulated termi'- nals 22a and 22h mounted on the end piece 20h. These terminals are connected by suitable wires, not shown to insulated terminals 23 mounted on base plate 2, see Figure 2, and terminals 23 are connected through a current adjusting dev ice to a suitable source of heating current. A tempera'- ture measuring element is mounted in the vicinity 5 of the carrier plates I4. and this element conveniently may be in the form of a therrnmouple 24 mounted on the plate jig as shown in. Figure 4. This thermocouple is connected by suitable connections to a pair of terminals 25 mounted on plate 2, and these terminals are connected to 'a suitable indicating instrument represented at-26.

A cylindrical heat shield 21, .preferably formed of sheet copper and closed at the upper end, is supported from the heating box 20 and surrounds this box and extends down to av point near the posts.

upper. end of cylindrical shield Illshown in Figure l. The purpose of the shield 2'I is to protect the bell I'from the heat of the plate heating unit, and to provide additional protection for the jar against the deposit of metallic film on the inner surface thereof during evaporation. The evaporation procedure is as follows: `The entire inner surface of the bell jar I, and the surfaces of all parts within this jar, must first be thoroughly cleaned of all contaminating materials. The carrier plates also must be thoroughly `cleaned before being mounted upon the plate jig. Care should be taken not to handle the plates by the hand after they have been cleaned.

It will be understood that filaments 4, 5 and 6 have previously been prepared. Each of these` filaments is formed of three strands of 2 0 mil tungsten wire plaited together at the central por- .tion and connected to the supporting terminal Around the plaited central portion of each filament is closely wound 24 inches of 1.0

` mil Nichrome wire.

A suitable quantity of p owdered magnesium fluoride is placed in each of the cups 'I and 6 before the shield I is placed around the terminal plate 3.

The carrier plates I4 and the test strip I6 are mounted upon the plate jig, and the jig is then placed in position at the top of the rods Za-Zd. The necessary connections are made from the test strip I6 and the thermocouple 24 to the appropriate terminals on base 2. The heater unit is now mounted on top of the plate jig and connected to the appropriate terminals on base 2, and' after this the heat shield 2'I is supported on the heater unit, then the jar I is placed in position on the plate 2 and the evaporation cycle may be started.

It will be understood that the plate 2 is provided with suitable connections not shown for vacuum gauges such as a Pirani gauge and an zionization gauge. The bell jar is first evacuated to. a pressure of 6 10-4 mm. or lower before cur- 'rent is applied to the heater filament 22. Evacuation is continued during heating of the interior of the jar by the heater 22, and when the temperalture within the jar rises to 350, as indicated by .the meter 26, the heater is turned off, and the Yevacuation is continued until the pressure drops to 6 or 7 105 mm. When this pressure is reached, the heater is turned on again, and when the temperature rises to 300 C., the heating current controlled to maintain the temperature with the jar not lower than 300 C. and not Ogreater than 350 C.

With the pressure at 6 or 7 105 mm. or less,

.1.3116 evaporation may be started by switching inI one of the vaporizing filaments 4, 0r6. The current in the filament is increased to a value sunicient to melt the Nichrome wire on the filament, and this required about amperes with the filament mentioned above. Nichrome clings to the vaporizing filament and completely surrounds the central portion thereof which operates at high temperature, and the molecular radiation from the Nichrome is de- "posited on the plate I4.

During the evaporation, the resistance of the test strip may be measured by the bridge I9, and since the resistance value of the test stripbears 'Va Vvery definite relation to the resistance value of the film deposited on the plates I4, the reading of the bridge I9 may be translated into indica- '.tions of the resistance of the films on the plates I4., If the desired resistance value is obtained in ,less than three .minutes of evaporation, the prjoc- The melted ess is stopped by de -energizing the filament when the resistance reading reaches the correct value. If the required resistance is lower than that which can be obtained in three minutes of evaporation, the filament is disconnected. vand a new filament is energized to continue the process until the ydesired resistance value is obtained. 4Care should be taken not to allow any single filament to continue operation beyond three minutes. While only three vaporizing filaments have been shown in the drawing, it will be understood that any desired number may be provided. During the evaporation of Nichrome it is important that the pressure within the jar I vnot rise to a value greater than 6 or 7x105 mm. It will be found that during actual evaporation of the Nichrome, the pressure will be of the order of 3 to 5 105 mm.

After the metallic film has been deposited, and While 4stillmaintaining thelow pressure lwithin the jar, one or both of the heater filaments are energized for cups 1 and 8 to vaporize the fluoride and deposit a protective coating on the metal films deposited on the plates I4. After all ofthe uoride has .been evaporated, the filaments are cle-energized. The vaporizing cycle is now at an end, and the pressure within the jar may be gradually increased until the jar may be opened for removing the coated plates.

For successful operation, it is necessary after each coating cycle to thoroughly clean the apparatus within the bell jar, and it is especially important to remove the film deposited on the vinside of the shield I0. This may be done by using vsteel Wool and a suitable cleaning powder or grease dissolving detergent. Other parts of the api paratus, including the interior of the bell jar,

should4 be wiped clean with a cloth wet with acetone.

For successful results, it is also important that a vaporizing filament shall not be used for more than one vaporizing cycle, .but a new filament with a new supply of Nichrome should be provided for each cycle, sincethe tungsten filament, once used, becomes very brittle and is apt to break upon reuse. Also, the molten nickel has dissolved some of the tungsten of the filament, and reuse of the filament would result in contamination of the deposited film by the tungsten. By providing a larger quantity of Nichrome than normal .on the vaporizing filament, the

4tungsten is coated with a Nichrome layer so thick that a satisfactory film can be formed before the dissolved tungsten has time to diffuse through the Nichrome layer to the surface.

It has already been indicated that it is im*- portant not-to continue the vaporizing cycle of any single filament for more than three minutes. The amount of vaporization permissible from a single filament may also be determined by the resistance value of the film deposited on the test strip. For example, when using a test strip of an inch wide and having a distance of 2 inches between the terminal coatings, the evaporation from a single filament should not be continued after the test strip attains a resistance value of 1000 ohms, assuming that the conditions of `evaporationare those indicated previously.

After the coated plates I4 have been removedv from the bellljar, suitable terminals may be attached to the end portions of the plates, thus 'adapting them for use'as resistance elements. It will be understood, however, that terminal members inthe form of low resistance coatings on the end `portions of the strips may be applied before the lms are deposited on the plates.

We claim:

1. In the manufacture of alloy resistance elements, the process which comprises heating a clean dielectric carrier to temperatures within the range of from about 300o to 350 C. in an enclosed space in which the pressure is less than 8 105 mm. of mercury, simultaneously heating a tungsten filament in said enclosed space in contact with a nickel alloy, selected from a group consisting of nickel-chromium and nickel-copper, under conditions causing the alloy to melt, to cover the filament and to vaporize, said dielectric carrier being sufficiently close to the filament to be coated by the vaporized metal, stopping the heating of the filament before any substantial contamination of the coating is produced by vaporization of tungsten from the lament, and vaporizing magnesium fluoride in proximity to the alloyfcoated carrier to coat said carrier with magnesium fluoride while still under a vacuum.

2. The process of claim 1 wherein vaporization of the alloy is continued for a period not substantially exceeding 3 minutes.

3. The process of claim 1 wherein the carrier is exposed to the vaporized metal until the metal iilm deposited thereon attains a thickness of at least 50 molecules.

4. The process of claim 1 wherein said tungsten filament is heated to a temperature of substantially 1400 C'. to vaporize said nickel alloy.

5. The process of claim 1 wherein the nickel alloy is vaporized from a plurality of tungsten filaments which are heated one at a time to vaporize the alloy therefrom.

6. In the manufacture of alloy resistance elements, the process which comprises enclosing a clean dielectric carrier within a closed space, maintaining a vacuum within said space measuring less than 8 105 mm. of mercury, heating said carrier at a temperature within the range of from about 300 degrees to 350 degrees C.. and

ysimultaneously heating a tungsten filament in said enclosed space in contact with a nickel alloy, selected from a group consisting of nickelchromium and nickel-copper, under conditions causing the alloy to melt, to cover the filament and to vaporize, said dielectric carrier being exposed to and coated by vaporized' metal radiated l from said filament, stopping the heating of the filament before any substantial contamination of the coating is produced by vaporization of tungsten from the iilament, and vaporizing magnesium fluoride within said space while maintaining said vacuum and temperature to coat said carrier with magnesium fluoride.

7. An electrical resistance element comprising a dielectric carrier having a metallic film deposited on the surface thereof from a vaporous state, said lm being formed of a nickel-chromium alloy having a lower temperature coeicient of resistance than its constituent elements, and a protective coating of magnesium fluoride covering said metallic nlm and deposited on said lm from a vaporous state.

8. An electrical resistance element comprising a dielectric carrier having deposited on the surface thereof from a vaporous state a nickel-copper alloy having a lower temperature coeicient of resistance than its constituent elements, and a protective coating of magnesium fluoride covering said metallic iilrn and deposited on said fihn from a vaporous state.

9. An electrical resistance element comprising a dielectric carrier having a metallic lm deposited on the surface thereof from a vaporous state, said nlm being formed of an alloy selected from the group consisting of nickel-chromium and nickel-copper having a lower temperature coefficient of resistance than its constituent elements, and a protective coating of magnesium iiuoride covering said metallic iilm and deposited on said llm from a vaporous state.

ERNST WEBER.. STANLEY ADAMS' JOHNSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,584,728 Case May 18, 1926 2,079,784 Williams May 11, 1937 2,210,308 Swinden Aug. 6, 1940 2,236,911 Long Apr. 1,v 1941 2,366,687 Osterberg Jan. 2, 1945 2,398,382 Lyon Apr. 16, 1946 2,416,211 Osterberg et al Feb. 18, 1947 2,427,592 Dimmick Sept. 16, 1947 2,432,538 Ogle et al Dec. 16, 1947 2,432,657 Colbert et al Dec. 16, 1947 FOREIGN PATENTS Number Country i Date 342,300 Great Britain Jan. 28, 1931 537,085 Great Britain June 9, 1941 5 Polytechnic Institute of Brooklyn, Report R-116- 45, Oct. 31, 1945. 

1. IN THE MANUFACTURE OF ALLOY RESISTANCE ELEMENTS, THE PROCESS WHICH COMPRISES HEATING A CLEAN DIELECTRIC CARRIER TO TEMPERATURES WITHIN THE RANGE OF FROM ABOUT 300* C. IN AN ENCLOSED SPACE IN WHICH THE PRESSURE IS LESS THAN 8X10-5 MM. OF MERCURY, SIMULTANEOUSLY HEATING A TUNGSTEN FILMENT IN SAID ENCLOSED SPACE IN CONTACT WITH A NICKEL ALLOY, SELECTED FROM A GROUP CONSISTING OF NICKEL-CHROMIUM AND NICKEL-COPPER, UNDER CONDITIONS CAUSING THE ALLOY TO MELT, TO COVER THE FILAMENT AND TO VAPORIZE, SAID DIELECTRIC CARRIER BEING SUFFICIENTLY CLOSE TO THE FILAMENT TO BE COATED BY THE VAPORIZED METAL, STOPPING THE HEATING OF THE FILAMENT BEFORE ANY SUBSTANTIAL CONTAMINATION OF THE COATING IS PRODUCED BY VAPORIZATION OF TUNGSTEN FROM THE FILAMENT, AND VAPORIZING MAGNESIUM FLUORIDE IN PROXIMITY TO THE ALLOY-COATED CARRIER TO COAT SAID CARRIER WITH MAGNESIUM FLUORIDE WHILE STILL UNDER A VACUUM. 